/*
* Copyright 2004 The Apache Software Foundation
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*     http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package javax.servlet;

import java.io.IOException;

/** 
* A filter is an object that performs filtering tasks on either the request to a resource (a servlet or static content), or on the response from a resource, or both.
* <br><br>
* Filters perform filtering in the <code>doFilter</code> method. Every Filter has access to 
** a FilterConfig object from which it can obtain its initialization parameters, a
** reference to the ServletContext which it can use, for example, to load resources
** needed for filtering tasks.
** <p>
** Filters are configured in the deployment descriptor of a web application
** <p>
** Examples that have been identified for this design are<br>
** 1) Authentication Filters <br>
** 2) Logging and Auditing Filters <br>
** 3) Image conversion Filters <br>
    ** 4) Data compression Filters <br>
** 5) Encryption Filters <br>
** 6) Tokenizing Filters <br>
** 7) Filters that trigger resource access events <br>
** 8) XSL/T filters <br>
** 9) Mime-type chain Filter <br>
 * @since	Servlet 2.3
*/

public interface Filter {

/** 
* Called by the web container to indicate to a filter that it is being placed into
* service. The servlet container calls the init method exactly once after instantiating the
* filter. The init method must complete successfully before the filter is asked to do any
* filtering work. <br><br>

     * The web container cannot place the filter into service if the init method either<br>
* 1.Throws a ServletException <br>
* 2.Does not return within a time period defined by the web container 
*/
public void init(FilterConfig filterConfig) throws ServletException;


/**
* The <code>doFilter</code> method of the Filter is called by the container
* each time a request/response pair is passed through the chain due
* to a client request for a resource at the end of the chain. The FilterChain passed in to this
* method allows the Filter to pass on the request and response to the next entity in the
* chain.<p>
* A typical implementation of this method would follow the following pattern:- <br>
* 1. Examine the request<br>
* 2. Optionally wrap the request object with a custom implementation to
* filter content or headers for input filtering <br>
* 3. Optionally wrap the response object with a custom implementation to
* filter content or headers for output filtering <br>
* 4. a) <strong>Either</strong> invoke the next entity in the chain using the FilterChain object (<code>chain.doFilter()</code>), <br>   
** 4. b) <strong>or</strong> not pass on the request/response pair to the next entity in the filter chain to block the request processing<br>
** 5. Directly set headers on the response after invocation of the next entity in the filter chain.
**/
public void doFilter ( ServletRequest request, ServletResponse response, FilterChain chain ) throws IOException, ServletException;

/**
* Called by the web container to indicate to a filter that it is being taken out of service. This 
* method is only called once all threads within the filter's doFilter method have exited or after
* a timeout period has passed. After the web container calls this method, it will not call the
* doFilter method again on this instance of the filter. <br><br>
* 
     * This method gives the filter an opportunity to clean up any resources that are being held (for
* example, memory, file handles, threads) and make sure that any persistent state is synchronized
* with the filter's current state in memory.
*/

public void destroy();


}

